Ion detectors based on a microchannel plate (MCP) or another type of the dynode electron multiplier are used in many applications, for example, as the ion detector of a time-of-flight mass spectrometer. A microchannel plate has an array of continuous-dynode electron multipliers. When an analyte ion is incident on the microchannel plate, the microchannel plate emits secondary electrons that are amplified by a cascade effect. The gain of the amplification depends on a bias voltage applied across the microchannel plate. The amplified electrons are subsequently converted to photons using a scintillator, the photons are converted back to electrons by a photomultiplier tube, and the resulting signal is digitized.
As disclosed in 215 INTERNATIONAL JOURNAL OF MASS SPECTROMETRY 13-30 (2002), the number of secondary electrons emitted in response to an incident ion depends on the velocity of the incident ion. In a time-of-flight mass spectrometer, the velocity of the incident ion is necessarily different for each species that arrives at the ion detector after a respective time of flight. Setting the bias voltage to provide a gain setting that maximizes the gain of the ion detector (and, hence, the detection sensitivity) for slow ions (i.e., ions having high mass-to-charge ratio (m/z)) can lead to reduced dynamic range and saturation in the data acquisition electronics for fast ions (i.e., ions having a low m/z). Furthermore, subjecting other system components to saturated signals can significantly shorten the lifetimes of such system components.
Accordingly, what is needed is an ion detector having a high gain for slow ions but that is not saturated by fast ions.